<!DOCTYPE html>
<!-- manifest="应用程序缓存清单文件的路径" 推荐文件扩展名为.appcache
就是一个文本文件 --> 
<html lang="en" manifest="demo.appcache">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        img {
            width: 300px;
            display: block;
        }
    </style>
</head>
<body>
    <img src="normal_2b.png" alt="">
    <img src="small_2b.png" alt="">
    <img src="super_small_2b.png" alt="">

</body>
</html>